Thermodynamics and Kinetics of Laser-Induced Structure Changes

Abstract

We suppose that a body is solid when its shear viscosity, η, exceeds the somewhat arbitrarily chosen value of 1015 poise, at which the time constant for shear relaxation would be about one day. 1 At temperatures below its thermodynamic crystallization temperature, Tm, a fluid will solidify heterogeneously by growth from seed crystals, or “nuclei”, to a crystalline body. In this process η is increased discontinuously, by many orders of magnitude, with the advance of the crystal-fluid interface. Alternatively, the fluid may, under certain conditions, solidify homogeneously by the continuous increase of η with falling temperature (or increasing pressure) into the solid range. Experience shows that the temperature, Tg, at which this solidification occurs is always less than Tm and that the solid body formed is amorphous in structure. Amorphous solids can sometimes also form discontinuously by condensation of material from fluids onto substrates held at temperatures below Tg.